1. Field of the Invention
The present invention relates to a high-frequency circuit element comprising resonators such as a filter or a branching filter for use in high-frequency signal processing devices used in communication systems.
2. Description of the Related Art
High-frequency circuit elements comprising resonators such as a filter, or a duplexer are essential in the field of high-frequency communication systems. In particular, the field of mobile communication systems requires a filter with a narrow bandwidth to efficiently use a frequency band. Further, in a base station for mobile communications or a communication satellite, a filter having a narrow band range, little loss, compact size and durability against a large electric power is desirable .
Conventional high-frequency circuit resonant filters comprise dielectric resonators, transmission line resonators, or surface acoustic wave elements. Conventional resonant filters comprising transmission line resonators are most widely used since they are compact, applicable to a high frequency as far as microwaves or milliwaves, and easily combined with the other circuits or elements to form a two-dimensional structure on a substrate. An example of a conventional resonant filter comprising a transmission line structure is a half-wavelength resonator which is most widely used. By connecting half-wavelength resonators plurally, high-frequency circuit elements such as filters can be formed ("Shokai Reidai Enshu Microwave Circuit" published by Tokyo Denki Daigaku Shuppankyoku).
Another conventional example is a resonant filter having a planar circuit structure. A typical example of a resonant filter having a planar circuit structure is one comprising a round planar resonator having a partially protruding portion at its circumference to couple dipole modes to display a filter characteristic (Institute of Electronics and Communication Engineers of Japan's article collection 72/8 Vol.55-B No.8 "Analysis of Microwave Planar Circuit" written by Tanroku MIYOSHI and Takanori OKOSHI).
However, resonators with a transmission line structure, such as half-wavelength resonators, have problems since high-frequency current tends to concentrate within the conductor to considerably increase resistance loss therein, which leads to the deterioration of the Q-value when used in a resonator or the increased loss when used in a filter. A half-wavelength resonator commonly used with a microstrip line structure has a disadvantage of radiation loss from the circuit.
Further, a resonator with a planar circuit structure comprising a round planar resonator with a protruding portion has electric current concentration in the protruding portion, and the discontinued structure at the protruding portion causes signal waves radiation to space, which will lead to the deterioration of the Q-value of the resonator, and the increased loss in this type of filter.
Such effects become more conspicuous if the structure is minimized or the operating frequency becomes higher. As a resonator of a comparatively little loss and good power handling capacity, dielectric resonators are used but the solid structure and bulkiness prohibits reducing the size of the high-frequency circuit elements.
Use of a superconductor can reduce the loss of such high-frequency circuit elements. However, in the above-mentioned conventional structures, superconductivity cannot be sustained in the above-mentioned conventional structure of a resonator due to the excessive concentration of the electric current. Therefore, it is difficult to use a signal of a large power. In the virtual measuring, the maximum input power is lower than 100 mW which is below a practical level.
With reference to the above-mentioned problems, obviously it is essential to solve such problems of resonators of a transmission line structure or a plane circuit structure to obtain a high-frequency circuit element including a resonant filter which has a compact and two-dimensional structure, matches other circuits or elements well, and performs excellently when applied to high-frequencies, such as microwaves or milliwaves.